The concept of extracting coherent optical radiation to produce a laser beam from a stream of free relativistic electrons has been known since the early 1970's. In most free electron laser applications, coherent radiation is produced when an electron beam is forced to pass through magnetic fields having different directions, which fields of different directions are produced by devices called "wiggler magnets." However, in one pass through the cavity only a tiny fraction of the beam energy is converted to coherent radiation, resulting in very low efficiencies. In order to improve efficiencies, efforts have been made to recover the energy of the electron beam remaining after its pass through the resonance cavity or to recirculate the beam for additional passes through the cavity.
At low electron beam energies (i.e., where the electrons in the beam have energies less than 10 MeV), most of the energy of the beam can be recovered by collecting the beam at high negative potentials. (See L. R. Elias, High-Power, cw, Efficient, Tunable (uv through ir) Free-Electron Laser Using Low-Energy Electron Beams, Phys. Rev. Lett., Vol. 42, Apr. 9, 1979, pp. 977-981.) The wavelength of the output laser is a function of the beam energy. An electron beam limited in energy to 10 MeV will limit the wavelength of the output laser to wavelengths of greater than approximately 20.times.10.sup.-4 cm.
The principal difficulty with recirculation of the beam is that a high efficiency output laser requires an electron beam with a very narrow energy spectrum, but the wiggler magnet process of extracting the coherent radiation reduces the magnitude of the energy of the beam and spreads the energy spectrum. Therefore, unless the quality of the recirculating electron beam can be improved between passes through the wiggler magnets, the efficiency of the output laser diminishes with each pass.
It is well known that the magnitude of a recirculated beam can be reinstated with an rf accelerator, and a combination of synchrotron radiation and additional wiggler magnets has successfully been used to reduce the energy spectrum spread of beams of electrons having energy in excess of 500 MeV. (See M. Billardon, et al., First Operation of a Storage-Ring Free-Electron Laser, Phys. Rev. Lett., Vol. 51, Oct. 31, 1983, pp. 1652-5.) The spreading of the energy spectrum is referred to as the "thermalization" of the beam, and the reduction of the spread of the energy spectrum is referred to as "cooling" the beam. Below 500 MeV, the magnetic fields required for this mechanism become too large to work effectively. A 500 MeV lower limit on electron energy limits the laser wavelength to less than about 10.sup.-4 cm.
Wavelengths between 10.sup.-4 cm and 20.times.10.sup.-4 cm are of interest because of their potential use in isotope separation and because of their transparency in the atmosphere. This range can be produced in free electron lasers using electron beams with narrow energy spectrums in the range of 10 to 500 MeV. What is needed is a method of cooling an electron beam having electron energies in the range above 10 MeV and especially in the range of 10 to 500 MeV so that the electron beam can be recirculated through a free electron laser in order to operate the laser at high efficiency.